The present invention pertains to the type of well drilling bit commonly referred to as a "roller cone" bit or "rock" bit. Such a bit typically includes a main bit body which is attached to and rotates with the drill string. The bit body includes a centralized portion, which has the joint for attachment to the drill string at one end, and also includes a plurality of radially and circumferentially spaced legs depending longitudinally from the other end of the centralized portion. A rolling cutter body or "cone" is rotatably mounted on the free end of each of these legs. Thus, as the bit body is rotated by the drill string, these cones are caused to roll along the bottom of the hole being drilled, and teeth on the cones disintegrate the earth formation.
Rotary bearings are provided between the cones and their respective legs, and these bearings must be lubricated. An annular seal is provided at the free or open end of the bearing interface between the cone and the bit leg in order to keep the lubricant in the bearing and exclude well fluids, and the abrasives they carry, from the bearing. It is very important that the integrity of these bearing seals be maintained. If it is not, the bearing can be ruined. This not only makes repair difficult and expensive, if not impossible, but drilling must be temporarily stopped, and the drill string tripped to replace the bit, an extremely expensive operation.
Accordingly, such bits are typically provided with a respective lubricant reservoir recess for each cone, and in this recess is disposed a mechanism which contains a supply of lubricant, which can be urged toward the bearing through an interconnecting lubricant passage, to keep the bearing supplied and replace any lubricant which is lost; the mechanism also includes a flexible, preferably elastomeric, pressure compensator such as an elastomeric diaphragm, one side of which is exposed to the lubricant pressure, and the other side of which is exposed to the pressure external to the bit in the borehole. The compensator can react to increases in external pressure, so as to ensure that the lubricant pressure is equal thereto, and prevent leakage of drilling fluid into the bearing. Conversely, it can react to decreases in well pressure to reduce the pressure it is exerting on the lubricant, to prevent diaphragm damage, lubricant waste, and/or displacement of or damage to the bearing seal.
There are two basic types of such mechanisms known in the art, distinguished by whether the vent port for exposing the diaphragm to the external borehole pressure opens generally upwardly through the outer part of the bit body, or downwardly, through the shroud of the bit body.
Examples of so called "top vented" pressure compensators are shown in U.S. Pat. No. 3,942,596 as well as in the commercial literature of assignee New Tech Rock Bit Co., and of Reed, Walker McDonald, and Rock Bit International, filed herewith. An advantage of the top vented lubricator/compensator mechanism is that it is relatively simple in terms of the number and nature of parts, especially seals, required. In the hostile downhole environment, it is generally the case that, the simpler a mechanism can be, and more particularly the fewer its parts, the less vulnerability to damage, deterioration by well fluids and/or abrasives therein, damage from temperature and pressure conditions, etc.
There is, however, room for even further simplification. For example, in the system shown in Assignee's commercial literature, filed herewith, the compensator proper, i.e. the diaphragm and a metal ring bonded thereto, are held in place by a cap, in turn releasably retained by a snap ring. The cap defines the vent porting. As can be seen in the illustration, the cap must be hollowed out in order to form a lowermost cylindrical projection which can fit over a portion of the aforementioned ring to abut a lower flange thereof for retention purposes, and also to seal against an O-ring carried in a groove formed in the O.D. of the ring. The bit body must also be machined to form a groove for holding an O-ring which seals against the outer surface of the cylindrical projection on the cap. Furthermore, the cap must be relatively long in order to provide this projection, which may affect the position, and thus the available size, of the lubricant reservoir space available for storing grease.